The present invention generally relates to a telemetry system for an implantable cardiac device. The present invention is more particularly directed to a telemetry system for an implantable cardiac device which monitors activity of a human heart, which also may additionally deliver therapy to the human heart, and which communicates with a non-implanted external device. The present invention is more particularly directed to a cardiac device which utilizes such a system to provide reduced power consumption while communicating data and command transmissions at a high transmission rate and with error correction and detection to assure accurate transmission even in noisy environments.
Implantable cardiac devices are designed to be small in size and implanted beneath the skin of a patient. Such devices establish electrical contact with the heart by one or more electrical leads having electrodes implanted within the heart, attached to the surface of the heart, or disposed subcutaneously and spaced from the heart.
Implantable cardiac devices include devices which monitor heart activity. Monitoring heart activity includes sensing electrical signals generated by the heart, recording data indicative of the sensed electrical signals and analyzing the recorded data to provide characterization data of the heart activity. Such devices detect arrhythmias of the heart, ischemia, or other conditions. The implantable device, under direction of a microprocessor, generates data indicative of these conditions and stores the data in memory. The data may be retrieved by an external device through telemetry.
In addition to monitoring heart activity, cardiac devices may also provide corrective therapy to the heart in the form of applied electrical energy. One such cardiac device is an implantable atrial defibrillator. Atrial defibrillators function to sense heart activity and deliver cardioverting or defibrillating electrical energy to the atria of the heart when the atria are in need of cardioversion. Control of these devices is provided by internal circuit means including devices such as a microprocessor. The microprocessor may be coupled to sensing means which sense the electrical activity of the heart and produce digital data corresponding to the sensed electrical activity. An atrial defibrillator implemented in the microprocessor in turn analyzes the digital data to detect conditions such as fibrillation of the atria. In response to detecting such a condition, the microprocessor directs other circuitry to apply defibrillating electrical energy to the atria of the heart to defibrillate the atria and return the heart to a normal rhythm.
Implanted cardiac devices such as these may receive command instructions from a non-implanted device such as a programmer which is external to the skin of the patient. These command instructions are referred herein as inbound transmissions, viewed from the perspective of the implanted device. The received command instructions may include program instructions or steps for directing the microprocessor's operation. The received command instructions may also include data such as program limits and timing data.
Similarly, the implantable cardiac device may transmit ECG or characterization data and other information to the external device. These are referred to herein as outbound transmissions, viewed from the perspective of the implanted device. The programmer may function to receive information from the implanted device as well as to transmit command transmissions including programming commands to the implanted device. Communication between the implanted device and the programmer may be limited to transmissions by only one of the devices with the other device receiving those transmissions. Alternatively, communication between the implanted device and the programmer may include transmissions by both devices.
Such communication is facilitated by receiving and transmitting means included in the implantable cardiac device and in the programmer. Where the implantable cardiac device includes a microprocessor, the receiving and transmitting means may cooperate with the microprocessor to receive command transmissions from the programmer and to transmit data and other information to the programmer. The programmer includes analogous transmitting and receiving means for communicating with the implanted cardiac device. Both the implanted cardiac device and the programmer include antenna means coupled to the receiving and transmitting means for transmitting and receiving electromagnetic energy.
Prior art devices typically enclose the antenna or antennas of the implanted cardiac device inside the enclosure of the implanted device. Such enclosures are typically metallic in nature and may be made of titanium. Metal enclosures act as low pass filters to limit the bandwidth of signals transmitted from and received by the implanted cardiac devices. Prior art designs which enclosed the antennas in the enclosure have experienced limited data transmission rates of less than 10K bits/second.
In prior art designs, bandwidth is also kept low in order to minimize the power consumed by the implanted cardiac device. Power consumption is a very important criterion in designing implantable devices. Such devices are typically powered by a depletable energy source such as a battery. A depleted energy source requires replacement of the cardiac device which can be costly and inconvenient and with potential risk to the patient. Accordingly, there is a need for an implanted cardiac device which exhibits minimized power consumption.
In accordance with the present invention, power consumption is minimized by de-energizing circuit components which are not currently required to be activated. Where the digital logic portions of an implantable cardiac device are fabricated in CMOS (complementary metal-oxide-semiconductor), this deactivation is inherent. Digital CMOS devices consume power only when switching, as is well known. Some portions of the circuitry of an implanted cardiac device are analog rather than digital in operation or are not fabricated in CMOS. Examples of this are the receiving means of the implanted device.
Both the receiving means and the transmitting means of an implanted cardiac device are used only intermittently to communicate with the programmer. After command transmissions are received from the programmer or after data and other information is transmitted to the programmer, the receiving means and transmitting means may remain inactive for extended periods of time. Accordingly, these circuits are well-suited to being de-energized when not in use in order to minimize power consumption.
However, when transmitting means and receiving means are de-energized to reduce power consumption, a further problem is created in that the implanted cardiac device is unable to receive command transmissions when its receiver is de-energized. Therefore, there is a need for activation means for activating the de-energized portions of the implanted cardiac device to allow the implanted device to receive command transmissions from the programmer.
One possible design for activating the device is passive wakeup. According to such a design, the programmer transmits a wakeup command with sufficient energy to convert the receiving means of the implanted device from a quiescent state, in which receiving and transmitting circuits are de-energized, to an active state, in which the implanted device and programmer are communicating. However, a relatively large receiving antenna is required in order to couple sufficient electromagnetic energy to the receiving means to facilitate this wakeup function. Such an antenna may be inconsistent with a physically compact implanted cardiac device or with an implanted cardiac device with the antenna in the header. The header is the epoxy or other non-metallic cover which sealingly engages the metallic enclosure of the implantable cardiac device. Some prior art devices use a large, flat antenna within the hermetic can. Even in a compact device, the antenna may be large enough that a wakeup command can be received by a quiescent circuit. However, once the antenna is made small enough to fit in the header, the need for an active circuit to receive the wakeup command becomes apparent. Therefore, there is a need for an implanted device having receiving means which may rest in a quiescent state and requires only a single, relatively small antenna while still providing reduced power consumption.
The present invention provides such a telemetry system. In accordance with the present invention, active wakeup is employed for activating de-energized portions of the implantable cardiac device. In accordance with this embodiment, the receiving means generally remains in a de-energized, quiescent state. At spaced apart time intervals, the receiving means is converted to an active state for a predetermined and relatively short time period in which it is energized and capable of sensing energy transmitted by the programmer. If no energy is sensed, the receiving means returns to the quiescent state. If energy is sensed, and the correct code is received, the transmitting means is energized to transmit a response code to the programmer. The response code indicates to the programmer that the implanted cardiac device is active and able to communicate. Accordingly, a single, relatively small antenna may be employed for both transmitting and receiving.
Such active wakeup in accordance with the present invention further allows for transmission of command transmissions at more than one data rate. Transmission of command transmissions at a low data rate allows the circuitry in the implanted cardiac device to operate at a low clock rate. As is well known, power consumption of CMOS digital logic devices is proportional to the clock rate of those devices. A low clock rate permits reduced power consumption. In accordance with the present invention, a low clock rate is used when the receiving means and transmitting means are in a quiescent state or when the receiving means is sensing energy from the programmer. Once the response code has been sent to the programmer and a communications link has been established, the low clock rate is converted to a high clock rate. Command transmissions are then transmitted at a high data rate consistent with the high clock rate of the implanted cardiac device. Transmission of data at a high data rate provides the further advantage of reducing power consumption by reducing the duration of time periods during which the receiving means and transmitting means must be in the active state.
In addition to power reduction, another important design criterion for implanted cardiac devices is accurate communication of data. Command transmissions transmitted from the programmer to the implanted cardiac device must be received correctly. Similarly, information including data transmitted by the implanted device must be received correctly. This communication must occur in environments such as hospitals and doctors' offices which may be very noisy due to the presence of other electronic equipment and other electromagnetic sources.
Therefore, there is a need for a telemetry system for implantable cardiac devices capable of transmitting and receiving data in noisy environments. Moreover, there is a need for such a telemetry system with the capability of detecting errors in data transmission and correcting those errors.
The present invention provides such a telemetry system. An implantable cardiac device in accordance with the present invention communicates with the programmer according to a communications protocol. Preferably, inbound transmissions are transmitted in accordance with a first protocol, and outbound transmissions are transmitted in accordance with a second protocol. According to both the first and second protocols, data is transmitted in packets. Each packet includes one or more frames. Moreover, each frame contains error detection codes used to detect bit-level errors. Each frame includes identifying information. In the event of such errors, only the frame or frames containing the errors need be re-transmitted (for the second protocol only). Thus, the ability to detect and correct errors without requiring the re-transmission of an entire transmission further reduces power consumption in such a telemetry system.
A further aspect of assuring accuracy of transmitted data is establishing a reliable data link. Generally, programmer antennas are disposed in moveable programmer heads which are to be placed in close proximity to the implantation site of the cardiac device. Because the implanted cardiac device is implanted and not visible, determining the proper orientation of the programming head of the external device can be difficult. To assure that data is transmitted accurately, the programming head antennas must be positioned to maximize signal strength received from the implanted cardiac device. Accordingly, there is a need for a telemetry system which provides an indication to the user of the relative received signal strength so that the antennas of the programming head can be positioned properly.
The present invention provides such a telemetry system. The telemetry system in accordance with the present invention includes a signal strength indicating means in the movable programming unit coupled to the programmer. The movable programming unit can be positioned external to the skin of the patient. An indication of received signal strength is provided to allow repositioning of the movable programming unit to maximize received signal strength.
In addition to assuring reliable communication of command transmissions and response information, another important design criterion for implantable cardiac devices is the ability to avoid locked-up devices after final manufacture of the device. The possibility exists that an implantable cardiac device may become completely inert after the final manufacturing step of welding the enclosure of the cardiac device. That is, the cardiac device may generate no output and not respond to telemetry transmissions. Accordingly, there is a need for a telemetry system including a hardware reset operation which will reset the cardiac device to a predetermined initial state. For this operation to function properly, the receiving means and transmitting means of the cardiac device must be able to operate independently of the microprocessor. The present invention provides such a telemetry system.